California, unlike Mississippi, is already on the road to Medicaid expansion under the Affordable Care Act. And after the law passed in 2010, it was the first state to get going to build an exchange.

Peter Lee is in charge of it. He never let uncertainty about the Supreme Court decision come in the way of building the new marketplace.

PETER LEE: We've been staffing up. We've been hiring people. We've been putting in place, in essence, all of the building blocks to expand coverage for Californians.

BARTOLONE: Lee left a job in the nation's Capitol. He says there, health care was a political football. In California, he's rolling up his sleeves in hopes the new health system will change lives.

LEE: I came home to California to be just thrilled by the amount of excitement and engagement that I've seen in every part of the state in making this exchange work.

BARTOLONE: And there's a lot of work to do. Lee has to build a Web system from scratch, and get insurance companies ready to offer health plans. He needs to recruit and train tens of thousands of people to help with enrollment and launch a huge marketing campaign.

ANTHONY WRIGHT: Our biggest obstacle is the clock. We don't have a lot of time.

BARTOLONE: Anthony Wright from Health Access California advocates for expanded health coverage in the state. Now he's going to push hard for the market to stay on track.

WRIGHT: We can't let this be like previous programs, where it starts slowly and builds gradually over time. We don't have that luxury.

BARTOLONE: Millions of Californians will be eligible to buy health insurance through the exchange. And Wright says it needs as many people under its umbrella as possible on day-one, 2014.

WRIGHT: Those who sign up the quickest will be the sickest, but it's going to work best if we have as many people signed up so that we can spread the risk and the cost, and that makes premiums lower for everybody.

BARTOLONE: The longer the exchange goes without broad enrollment, Wright says, the more the state loses out on federal dollars, and the more uninsured people may be living sicker and dying younger. The scope of the job is a challenge. One in five Californians is uninsured.

WRIGHT: Hopefully, we can show that if California can do it - given how diverse the state is, given how big the state is, given the size of our uninsured population - then that can prove that it can be done in any state.

BARTOLONE: Other states will surely be watching California. Peter Lee of the Health Insurance Exchange says he's confident the state will be ready to cover millions in just a few years. For NPR News, I'm Pauline Bartolone. Transcript provided by NPR, Copyright NPR.